TSETSE-TRYPANOSOMIASIS CHALLENGE TO VILLAGE-NDAMA CATTLE IN THE GAMBIA - FIELD ASSESSMENTS OF SPATIAL AND TEMPORAL PATTERNS OF TSETSE-CATTLE CONTACT AND THE RISK OF TRYPANOSOMIASIS INFECTION

The severity of the trypanosomiasis problem in a particular location i s traditionally assessed in terms of a challenge index - the product o f some measure of tsetse abundance and infection rate - which is assum ed to be proportional to the force of infection. However, this index m asks variation in the force of infection between herds and among indiv iduals within herds. It is also not comparable between sites since the relative abundance of tsetse to hosts map vary. We have studied spati al distribution of herds of cattle in relation to tsetse in The Gambia and calculated an index of challenge based on the ratio of vectors to hosts over the livestock ranging area. This index is strongly correla ted with estimates of the force of infection calculated from the incid ence of infection in susceptible zebu; and it provides information on heterogeneity in exposure of different herds to tsetse.